The present invention relates to a threshing device particularly for tobacco leaves.
In the processing of tobacco it is currently known to use devices which have the function of breaking tobacco leaves, thereby separating the laminar part of the leaves from the ribs or stems; said laminar part assumes the form of particles of lamina termed "strips".
This operation is indispensable since it is the lamina which is subsequently used in the manufacture of tobacco products and must therefore have the greatest and most uniform possible size.
Some known devices use a rotor which is provided with a series of radial blades arranged on diametrical planes, which will be referred to hereafter as dynamic blades.
Such known devices furthermore have a series of fixed blades associated with a static blade holder fixed to the framework.
Advantageously, the fixed blades are arranged on an axis which is diametrical to the rotor so that each series of dynamic blades passes cyclically between two flanking fixed blades.
A grid or a series of grids is provided below said rotor. The rotor usually rotates at a speed of approximately 400-450 rpm and may have six blades on each diametrical plane, whereas the leaves are introduced, by gravity or by pneumatic conveyance, at the region overlying the fixed blades.
The operation of said known devices is aimed at causing the leaves to become arranged transversely between two fixed blades, and then threshed by a dynamic blade in order to mechanically separate the lamina from the rib. The threshing action is completed by rubbing the leaves against a grid, also known as "basket", which is arranged below the rotor.
These known devices, however, are not devoid of disadvantages, not least of which is the fact that perfect placement of the leaves between the fixed blades is very rarely achieved in practice. This results in some leaves not being threshed, while other leaves are subjected, due to the rotation rate of the rotor, to multiple threshings which break up the lamina excessively, thereby reducing the size of the strips obtained.
This is a serious disadvantage because, in order to obtain a high quality product, it is preferable to have the largest and most uniform possible size of strips. In fact, at the threshing region between rotating blades and fixed blades, the condition occurs in which strips already separated from the ribs are undesirably forced to undergo the continuous threshing action of the subsequent rotating blades, because the fixed blades tend to oppose the downward outflow of the strips and ribs, thus causing the very harmful reduction of strip size and loss of uniformity.
Once it has left the threshing region, the product should be completely expelled through the grid.
However, a part does not escape and continues the rotation induced by the rotor.
This product, retained by the grid, undergoes a further threshing action at the rear static blades, if these are adopted, or continues to rotate until it is threshed again at the static blades.
This causes even further undesirable breakage of the strips, which deteriorate and lose uniformity.
In known devices, the use of ducted air flows at the region where the leaves are fed to the rotor has the mere function of conveying the leaves, and said ducted air flow does not prevent the leaves from undergoing numerous threshings.